The invention relates to a packaging casing in the form of a shirred stick based on synthetic or natural polymers, in particular an artificial sausage casing which is shirred from tubular material, and to a process for producing a shirred packaging casing.
Numerous plastic films already exist for packaging foods which can be packaged in pasty or molten form. Generally, they are produced on the basis polyethylene terephthalate (PET), polyvinylidene chloride (PVDC) and polyamide (PA). Biaxially oriented PET casings, however, are not suitable for cooked-meat sausage and scalded-emulsion sausage, since they have low extensibility and low shrinkage. Although PVDC casings are significantly more extensible, after stuffing they readily lose their shape. This is particularly marked if the sausages are suspended for a relatively long time. Polyamide casings are more extensible, shrink more and are therefore more suitable for cooked-meat sausage and scalded-emulsion sausage.
GB-A 2 035 198 (≈DE-A 28 50 182) describes a shrinkably stretched in the longitudinal and transverse direction and heat-set single-layer casing made of an aliphatic polyamide whose glass transition point in the dry state is at least 48xc2x0 C. and decreases on moisture absorption to at least 3xc2x0 C. Polymers which are explicitly disclosed are nylon 6 (=polycaprolactone), nylon 7 [=poly(7-aminoheptanolactam)=poly("khgr"-enantholactam)], nylon 6,6 (=polyamide of hexamethylenediamine and adipic acid) and polyamide 6,10 [=polyamide from hexamethylenediamine and decanedioic acid (=sebacic acid)].
A similar casing is described in GB-A 2 035 198 (≈DE-A 28 50 181). It consists of a polymer blend which, in addition to the aliphatic polyamide, also comprises an ionomer resin, a modified ethylene/vinyl acetate copolymer and/or a quaternary polymer containing units of ethylene, butylene, analiphatic, ethylenically unsaturated (C3-C5)carboxylic acid and an ester of this carboxylic acid with (C1-C8)alcohols. After incision, the casing shows a decreased tendency to tear propagation. These casings are also used for cooked-meat sausage and scalded-emulsion sausage. However, the sausagemeat emulsion must be stuffed at a pressure of about 0.3 to 0.6 bar in order to extend the casing.
Since the sausagemeat emulsion after scalding loses volume, the casing must compensate for this loss so that it lies against the stuffing in a wrinkle-free manner, that is to say it must shrink to the same extent as its contents. The known casings, however, are only shrinkable with limitations and are therefore extended during the stuffing operation. The necessary stuffing pressure is at the boundary of that possible using known sausage stuffing machines. With manual stuffing, the pressure is so low that such casings will detach from the sausagemeat emulsion and wrinkles would form, which would make the sausage appear xe2x80x9cunattractivexe2x80x9d and xe2x80x9cno longer freshxe2x80x9d in the eyes of the consumers.
A single-layer sausage casing made of a polyamide which can adsorb at least 5% of its weight of water is disclosed in U.S. Pat. Nos. 4,560,520 and 4,601,929 (≈DE-A 32 27 945. It has less of a tendency to propagate tearing after incision than the sausage casing disclosed by DE-A 28 50 182. Starting materials for this casing are in particular nylon 6 and nylon 6,6. It is produced by extrusion, stretching the extruded tube in longitudinal and transverse direction and then complete heat setting with controlled shrinkage. The controlled shrinkage in each direction is 15% to about 40%, generally about 20%. Here also, the casing must be expanded by elevated pressure during stuffing, if it is to remain wrinkle-free later.
The mechanical properties of stretched polyamide casings depend greatly on their water content. A minimum water content is necessary in order to keep the casing sufficiently extensible and supple during the stuffing process. However, water and water vapor increase this water content during scalding, so that the mechanical properties of the casings are impaired.
A single-layer polyamide-based casing for cooked-meat and scalded-emulsion sausages which remains dimensionally stable even during scalding is disclosed in U.S. Pat. No. 4,659,599((=EP-A 0 176 980). It consists of a blend of polyamide and polyester. Polyamide is a saturated, linear, aliphatic polyamide, in particular nylon 6, and the polyester is a polyterephthalic ester and/or a copolyester containing terephthalic acid and isophthalic acid units. Owing to the polyester admixture, the casing is relatively opaque and exhibits an unnatural metallic gloss. In addition, it is only slightly extensible and must be stuffed under a relatively high pressure if it is to remain wrinkle-free.
The single-layer biaxially oriented and heat set sausage casing according to U.S. Pat. No. 5,326,613 (≈DE-A 39 43 024) is glass clear, does not have a metallic or mother-of-pearl-like sheen and has a high rebound elasticity. It is produced from a polymer blend consisting of polycaprolactam (=nylon 6) and an aromatic copolyamide of hexamethylenediamine, terephthalic acid and isophthalic acid units. This casing is also primarily intended for machine stuffing under pressure. It is less suitable for manual stuffing, because it is generally not supple enough.
EP-A 0 573 306 describes a tubular multi-layer biaxially oriented shrinkable sausage casing. The casing comprises one layer of a polymer blend consisting of nylon 6 and at least 5% by weight of a copolyamide (for example nylon 6,12 or nylon 6/69). This layer is surrounded by other layers, especially polyolefin layers. The polyamide layer is relatively thin compared with the polyolefin layers. The food casings explicitly disclosed in the EP-A are all of six layers.
U.S. Pat. No. 5,928,738 (DE-A 196 25 094 A1) discloses a polyamide-based biaxially stretch-oriented and heat set single-layer or multilayer shirred stick packaging casing which has a shirring density of up to 1:200 and the length of the shirred stick is 40 to 100 cm, and a process for producing such a packaging casing. The wall thickness of the packaging casing is in a range from 33 to 55 xcexcm. Before shirring this packaging casing a spray solution is applied which, as shirring lubricant, comprises an emulsifier, paraffin oil or similarly acting material.
DE 297 14638 U1 describes an apparatus for shirring sausage casings having a screw-nut-shaped shirring element of oxide ceramics. The tooth flanks and the points of the shirring element have a surface finish of Raxe2x89xa60.3 xcexcm. Using this apparatus, sausage casings can be pushed together in a shir-like manner, the shirred sticks having a length of 40 to 50 cm. The shirring element in the form of a screw nut is fixed to a housing which is mounted so as to be able to rotate with the aid of ball bearings which are disposed in a stand. The housing is driven by a toothed belt by means of a speed-controlled electric motor. Using oxide ceramics for the shirring element achieves a considerable improvement in the service life of the shirring element.
In the case of known packaging casings in the form of a shirred stick, it is found that from a shirring density of 1:200, the sliding friction between a shirring rod and the packaging casing in the form of a shirred stick seated on this shirring rod is so great that fault-free end processing can only be carried out with difficulty. Shirring holes can occur in the packaging casing in the form of a shirred stick or else damage to the shirring rod can occur. The resultant high sliding friction is due to the high adhesion forces between the smooth contacting surface of the shirring rod and the very smooth tubular material. The applies in particular in the event that the shirring rod is coated with Teflon which has a very smooth surface which then together with the smooth tubular material of the polymer casing gives very smooth contacting surfaces, between which high adhesion forces occur. In the case of the known packaging casing in the form of a shirred stick the problem occurs to the extent that shirring densities greater than 1:200 make end processing of the packaging casings in the form of the shirred stick very difficult.
It is an object of the invention to provide a packaging casing in the form of a shirred stick of greater shirring density and greater shirred length than known packaging casings in the form of a shirred stick which make it possible to end-process and pull off the shirred packaging casing from a shirring rod without damage.
This object is achieved in such a manner that the roughness of the tubular material in contact with a shirring rod and the roughness of the shirring rod together have a mean roughness of 0.5 to 5 xcexcm, the shirring density is greater than 1:200 and is up to 1:500, and the shirred stick is 10 to 200 cm long at a wall thickness of the tubular material of 20 to 80 xcexcm.
In an embodiment of the invention, the wall thickness of the tubular material is 20 to 55 xcexcm. Expediently, a spray solution is applied to the tubular material before shirring, and the spray solution contains a lubricant. The use of emulsions as shirring lubricant is not absolutely necessary, rather the use of standard lubricants, as are known to those skilled in the art, is generally sufficient.
Expediently, the tubular material in the unshirred state has a caliber of 20 to 200 mm, in particular 40 to 140 mm.